Is there an unified chart for how much favor you need with a god to increase your level with him/her?
IME reaching up to level 3 with Sabine was very easy, but now I'm stuck in level 3 for quite some time, and my favor is almost 4000.

I have also plateaued and given up only to find later that I gain several levels.

I almost always play as a lay worshiper. I lose the ability to pray anywhere, and hallow spell is unusable, But I can pretty much worship everyone that tolerates me.
Things I've noticed:
you only ever gain levels when you sac ( if you figure out how worshiping Asherath works, please let me know.
you can only ever gain one level per sac
you can gain levels even if you're sac doesn't net any favor: if you donate 5k gold to Hesani, you can gain a few levels more by donating 1 gold several times.
Eventually saccing wont get you any more favor (unless you find a very good sac). but you can still gain favor by doing things the gods like Then saccing for the level up. (I think this is how this works)
Ekliaz is hard for me to get to level 2 sometimes, but if I come back after dungeon level 5 or so, I can usually get to level 4 as a human( I think because you kill so many gobs and stuff.)
Knowledge theology can make your life easier. Sometimes I take human or natural aptitude just for this.

Most gods take a set number of sacs. Once those 'slots' are filled additional sacs replace a previous sac.
So if I donate 250 gold 4 times to hesani, I get 1k gold worth of favor with Hesani, but when I donate later, I'll lose one of those donations. That gold is gone and worthless now.
If I donate 995 gold once, and 1 gold 5 times, then I have 1k gold worth of favor still, but when I donate again later, I'll only lose 1 gold. Overall netting more. In short, 1 large donation is usually better unless you need the favor now. FYI I don't actually know how many 'slots' Hesani, or any other god, has.